70, fit, BMI 21, tummy tuck & past lipo, I don't know if I should have muscle tightening? (Photos)

Yes I recommend you have muscle tightening with your tummy tuck as it will give you a much better result.
There are two major steps in tummy tuck (and many important minor ones) - removal of excess skin and tightening the muscles and you will benefit from both of these steps, in fact I wouldn't recommend doing the tummy tuck without the muscle tightening in your case, as you demonstrate very nicely in your photos the difference it will make for you.
I would have no concern doing a tummy tuck for you 20 years after liposuction, The scars in the areas treated with the liposuction will be soft now and won't present a problem. You have a smooth tummy now and can expect a smooth tummy after, unless you have a complication (for example, infection). Yes experienced plastic surgeons deal with this all of the time.
The only challenge compared to an average tummy tuck is that you are 70, and as we age, we don't bounce back from things as quickly as when we are younger. However, the fact that you exercise and have a BMI of 21 also goes a very long way to predict that you will have a good recovery. This comment on your age is a scientific fact, and not a judgement. My advice is make sure you get enough rest after surgery yet be sure to be up and about every day, and I expect you will do great.
I think it will be a good idea for you to do some more consults.
All the best to you,
Dr. Nancy de Kleer

Based on the pictures you provided you can be a candidate for second surgery. However, a detailed physical examination would help you out better.

Yes we deal with this all the time and yes it is more difficult and the results less predictable when you have had lipo in the past. Your PS is correct on the recommendations. You don’t appear to have much laxity in the lower abdominal skin and maybe consider a reverse tummy tuck.